I will defend my thesis soon. It would be great if someone could suggest me any good course or resource to improve my skills.
I recommend Presentation Zen, especially the book. Even though it stays on a fairly high level (and is definitely tailored towards business & marketing), its main ideas are very useful for academic presentations.
I highly recommend book "Designing Science Presentations: A Visual Guide to Figures, Papers, Slides, Posters, and More" by Matt Carter. It is very comprehensive in covering various aspects of scientific presentations in different contexts. Another excellent book on the topic is "Presentation Secrets" by Alexei Kapterev. I hope to use advice from both books in my future academic career.
Unfortunately, I have bought both books after I've almost prepared my dissertation defense slides, so I haven't had a chance to improve my slides, based on solid advice in the books (that is not to say that my slides were bad without it, but pursuit of perfection is one of my core problems :-). Therefore, I have just browsed both books in search of advice for other presentation aspects (speaking, equipment, etc.). Having said that, I think that the single most important factor of success for a dissertation (thesis) defense is your confidence (in both yourself and your work).
Another very important factor is timing. You have to pay a lot of attention to timing, since in many, if not all, cases dissertation defense is quite limited time-wise (mine was very limited). After working for years on some subject domain, you have a lot to say and, thus, compressing what you want to say into what you must say represents a major challenge. The solution to that is to practice. I have literally recorded my voice presentation on my smartphone and trimmed it (and correspondingly, my slides) until the length of the presentation was approaching the target time frame. I also played my recordings before sleep for better memorization (for fluidity). Finally, I asked a friend, who was also finishing working on his Ph.D. dissertation, to do me a favor and listen to my presentation via Skype (sharing the screen for synchronized slide presentation). We did that twice. I have listened to his feedback and updated my slides and talk plan, correspondingly. Some stress is inevitable, but you can definitely minimize it to a tolerable level, especially considering that the fact of your defense being scheduled implies that your work has been accepted by your committee and, thus, they have enough confidence in you. I hope that my answer is helpful. Good luck with your defense!
Matt Might has a nice article on tips for academic talks, with some very helpful tips. He is in Computer Science, so some things may be more or less applicable to your field. In that article he also recommends "Even a Geek can Speak" which I have read and highly recommend as well. I believe if you click around his website you can find some other tips on academic talks, and on presenting academic work.